1. Field of the Invention
This invention relates to a process for modifying polymers during and after fabrication. More particularly, this invention relates to a process for improving the thermal, optical and/or mechanical characteristics of polymers.
2. Prior Art
X-ray diffraction studies of synthetic linear is polyamides such as nylon 6 have shown that they may exist in one or more of several different crystalline forms. Structures which have been recognized include, in the case of nylon 6, the amorphous form, the pseudo hexagonal gamma-form, and the monoclinic alpha form.
The amorphous form of nylon 6 is obtained by rapid quenching of molten polymer to a temperature below the glass transition temperature of the nylon. Both the amorphous and gamma-forms are relatively unstable upon application of heat and moisture. Heating amorphous material to a temperature between approximately 55.degree. C. and 150.degree. C. results in at least a partial conversion of the amorphous form into the gamma-form. At temperatures above 150.degree. C., a transition of the gamma-form into the alpha-form occurs. This monoclinic alpha structure represents a highly ordered crystalline form that is stable at temperatures up to the melting point of the nylon 6. It is the most desirable crystalline form from the standpoint of obtaining the optimum physical properties with nylon 6, including mold shrinkage and maximum dimensional stability.
The "super" or morphological structure in which the crystalline units are arranged also affects the physical properties of nylons. The crystalline units are arranged in polycrystalline aggregates known as spherulites. These spherulites may be detected by microscopic examination under polarized light. They are characterized by a more or less symmetrical growth in all directions from a nucleus and are composite structures made up of crystalline and amorphous regions. The number and size of the spherulites determines the texture or graininess in the bulk of the material and influences optical as well as physical properties. Physical properties improve with increasing homogeneity and fineness of the spherulitic structure throughout the bulk of the material.
To obtain optimum physical properties in articles fabricated from nylon 6, it is desirable, therefore, to produce a highly crystalline material, crystallized predominantly in the stable alpha-form, with an extremely fine, dense and uniform morphological structure. Among the physical properties affected by increased crystallinity and improved morphological structure are abrasion resistance, heat distortion temperature, inherent stability or resistance to deformation, resistance to hot water, coefficient of expansion, hardness, yield, tensile strength and surface hardness.
Customary fabricating procedures used with nylon 6 such as injection molding, which include a rapid cooling from the melt, generally result in articles which contain the different crystalline structural forms to a varying degree depending upon the thermal history of the article.
It is known that a greater degree of crystallinity is obtained when polyamides are cooled extremely slowly from the melt; however, under these conditions, large spherulites develop which is not desirable. Moreover, the process is not economical. Crystallinity and the uniformity of the morphological structure can also be increased by annealing treatments after solidification. However, such practices are not economically feasible in ordinary industrial fabricating procedures as, for example, injection molding. More recently, investigators have found that bodies shaped from polyamides having a homogeneous and fine spherulitic structure can be obtained by addition to the polyamide melt of finely divided particles which act as crystallization nucleic (e.g., U.S. Pat. Nos. 2,855,377; 3,261,800 and 3,080,345; Belgian Patent 674,170; Japanese Patent Nos. 52-105958 and 80-041623, U.S. Pat. No. 4,501,844; Gurato, G. et al. "Influence of Nucleating Agents on the Crystallization of Nylon 6", Makromol Chem., Vol. 179(1), pp. 231 to 245 (1978); Chatterjee, A.M., et al., "Heterogeneous Nucleation of Crystallization of High Polymers From the Melt", J. Polym. Sci,. Polym. Phys. Ed., Vol. 13(12), pp. 2368-83 (1975); and Koutsky, J. A., "Nucleation of Polymer Droplets", J. Appl. Phys., Vol. 38(4), pp. 1832-39 (1967).
The function of nucleating agents when cooling semicrystalline polymers from the molten into the solid form is to increase the number of nuclei formed in a given time interval at a predetermined temperature. The final and over-all crystallinity, however, depends not only on the number of nuclei that are formed but also on the spherulitic growth rate from such nuclei. As noted above, spherulites develop with respect to a center, or nucleus, of growth. Addition of the nucleating agents thus provides a large number of sites for growth upon cooling from a melt. In order to be of practical use, such nucleating agents not only must produce a fine spherulitic structure but also must do this under conditions of rapid cooling to a temperature above the glass transition temperature of the polyamide, i.e., they must reduce the time that is necessary under a given set of conditions for crystallization to start. This time is usually referred to as "induction time". Subsequent growth from the spherulitic center depends on the polymer chain mobility. Thus, a factor in the spherulitic growth rate is the macroscopic viscosity of the polymer and its temperature dependence. All segmental motion is "frozen in" at the glass transition temperature (Tg) and no additional crystallization occurs even when nuclei are present. This Tg is about 50.degree. C. in nylon 6.
Other prior art describes other additives which can be used in addition to nucleating agents to enhance their effectiveness. For example, U.S. Pat. No. 3,549,651 discloses that the effectiveness of nucleating agents for linear polyamides in terms of the rate of crystallization into the desirable crystalline form can be increased by using a plasticizer in conjunction with the nucleating agent. U.S. Pat. No. 4,200,707 discloses the use of zinc phenyl phosphinate and zinc phenyl phosphate as nucleating agents for certain polyamides, and U.S. Pat. No. 3,867,339 describes polyamides containing sodium phenyl phosphinate and a calcium salt. U.S. Pat. No. 4,397,979 discloses that materials such as lithium metasilicate, lithium aluminate, lithium metaphosphate, lithium fluoride, magnesium fluoride, zirconium tetrafluoride, zirconium silicate and mixtures thereof. Great Britain Patent No. 1,211,689 and German Patent No. 1,694,476 disclose that substances such as calcium fluoride, molybdenum disulfide, lead tungstate, antimony trioxide, graphite, talcum and asbestos are effective nucleating agents. U.S. Pat. No. 4,159,286 discloses the use of polytetrafluoroethylene and at least one water insoluble organic salt or inorganic salt or oxide as the nucleating agent. U.S. Pat. No. 3,645,932 describes a process of developing rapidly the crystalline characteristics of certain polyamides by incorporation of a variety of nylon and other polyamide resins, polyethylene, polypropylene, polycarbonate and polyethylene acrylic acid copolymer ionomers. U.S. Pat. Nos. 4,536,533 and 4,501,844 disclose a rapidly crystallizable composition comprising (a) a linear polyamide selected from the series poly(4,4'-methylenediphenylene azelamide to dodecanediamide); and (b) an amount sufficient to promote the crystallization of the linear polyamide, of a material selected from the group of (I) talc, (II) sodium benzenesulfonate, (III) a polyethylene ionomer, (IV) a methacrylated butadiene-styrene, and (V) a multi-phase composite interpolymer. The compositions crystallize rapidly from the molten state which allows for economically attractive molding procedures including fiber production and the production of aromatic-aliphatic polyamides having a combination of excellent properties.
U.S. Pat. No. 3,400,087 discloses that the rate of crystal growth without substantial increase in the percent of total crystallization of polyamides is obtained by incorporating in the melt finely divided calcite particles and a fatty alcohol. DD Appln. No. 103,456 describes the use of nucleating agents for polyamides, especially tannin. Japanese Patent No. 73-017552 discloses a composition comprising a polyamide, e.g., nylon 6, 11, 12, 66, 610, etc.) and 0.01-3 wt % of a compound selected from As-oxide, Sb-oxide and Bi-oxide, talc and clay, and 0.01-3 wt % of either a polyvalent alcohol (e.g. trimethylolethane, trimethylolpropane, erythritol, sorbitol, amnnitol, etc.), or an aromatic ketone (e.g.) benzophenone, phenyl benzyl ketone, dibenzoyl, benzoyl methane, etc.).
U.S. Pat. No. 3,645,932 discloses nucleating agents selected from: (a) polymers and copolymers of monolefins, having from 2 to 6 carbon atoms the ionomer of ethyleneacrylic acid copolymer; (b) a polyamide of 1) epsilon-caprolactam, 20 omega-amino undecanoic acid, 3) omega-aminododecanoic acid, 4)hexamethylene diamine and adipic acid, 5) hexamethylene diamine and sebasic acid, or a copolyamide resin derived from polymeric fatty acids, sebasic acid and bisamino-ethylbenzene; (c) polycarbonate of p,p-isopropyliodenediphenol and phosgene; (d) polystyrene terpolymers of acrylonitrile, butadiene and styrene; (e) natural rubber, balata; and (f) paraffin wax, microcrystalline wax, are dispersed in an amount of 0.1-10 wt % in a crystalline polyamide derived from hydrogenated polymeric fatty acid, a coacid having from 6 to 10 carbon atoms and a diamine, and 25-75 wt % of the polyamide derived from the reaction product of the coacid and diamine.
U.S. Pat. No. 3,549,651 describes a solid synthetic linear polyamide composition having enhanced crystalline uniformity, the polyamide having dispersed in it about 0.10-5.0 weight percent of a finely divided nucleating agent selected from a group consisting of sodium phenyl phosphinate, sodium isobutyl phosphinate, silver bromide, molybdenum disulphide, boron nitride, the sodium complex of phthalocyanine and talcum, and from about 0.25-20 weight percent of a plasticizing agent selected from the group consisting of triethylene glycol, 2-ethyl-1,3 hexane-diol, tricresylphosphate, N-ethyl ortho- and para-toluene sulfonamide, and a mixture of N-ethyl and p-toluene sulfonamide.
U.S. Pat. No. 4,518,731 discloses that an adduct of aryl carboxylic or sulphonic acid and crystalline zeolite functions as a nucleating agent for crystalline polymers.
U.S. Pat. No. 3,367,936 describes a nucleating agent for crystalline copolymers selected from poly-alpha-olefins, thermoplastic polyesters, and polyamides, which comprises an adduct obtained by interaction of (a) an arylcarboxylic acid or sulphonic acid chosen from those which in the form of metal salts are known to exert a nucleating effect on the crystalline polymers of alpha-olefins, the acid having a melting point greater than the crystallization temperature of the polymer and being thermally stable at the maximum operational temperature in processing of the polymeric material, and (b) a crystalline aluminum silicate of an alkaline or alkaline earth metal of the zeolitic tectosilicate type, having a pore diameter of 3 .ANG.-3 .ANG. and a molar ratio SiO.sub.2 /Al.sub.2 O.sub.3 =2-5, in powder form, under such conditions that allow chemi-absorption of the acid (a) in component (b) such that 1-25 g (a) is present per 100 g (b) and (a) is partly bound to (b) by chemical bonds of polar type.
DD Patent No. 151,317 describes the fact that polymerization of alpha-crystalline Epsilon-polymerization shaped objects stabilized by additives involves the addition by weight of 0.01-0.2% CuCl 0.02-0.5% KBr, 0.2-2.0% melamine, 0.01-0.5 (0.05-0.2)% talcum and 0.1-2.0% of a metal salt of a 12-20 carbon monocarboxylic acid (preferably 0.2-1.0 wt % calcium or zinc stearate) to the caprolactam, as well as any pigments, fillers or reinforcing agents that may be required. The additives serve to stabilize the product.
U.S. Pat. No. 3,759,221 describes a polyhexamethyleneadipamide composition which contains an inert nucleant, a specified alkylene diamide and a specified metal carboxylate. It is disclosed that this material is fast cycling.
U.S. Pat. No. 4,799,736 describes a crystalline polyamide composition having dispersed therein a nucleating agent composed of one or more partly divided inorganic materials, one or more fatty acid amides and one or more low molecular weight polymers selected from the group consisting of polyolefins, polyoxides and polysulfides. It is disclosed that the nucleating agent provides for a relatively homogeneous and fine spherulitic or crystal structure.